Interactive Evolutionary Computation
An interesting subject inside the Evolutionary Computation field is the so called Interactive Evolutionary Computation (IEC). According to Wikipedia, IEC "is a general term for methods of evolutionary computation that use human evaluation. Usually human evaluation is necessary when the form of fitness function is not known (for example, visual appeal or attractiveness) or the result of optimization should fit a particular user preference (for example, taste of coffee or color set of the user interface)".
Maybe, the main problem that IEC faces is the human fatigue. As computer evaluations of fitness functions are faster than those made by humans, that means that only few evaluations of the fitness function can be made through humans without causing to those the fatigue. Although there are some strategies which can be applied to avoid that, such as small number of fitness function evaluations and friendly human-computer interfaces.
Some examples of IEC are:
Karl Sims' Galápagos.
SBART developed by Tatsuo Unemi from Soka University, Japan.
Evolution through subjective selection by Michael Herdy from Technical University of Berlin (This last one is a flash animation which you can download to your computer and run in your browser.)